Partially hydrolyzed aluminum alkyl compounds, known as aluminoxanes (also called alumoxanes), are effective in activating metallocenes for polymerization of olefins. Methylaluminoxane (also called methylalumoxane) has become the aluminum co-catalyst of choice in the industry. It is available commercially in the form of 10 to 30 wt % solutions in an aromatic solvent, typically toluene.
Considerable effort has been devoted to improving the effectiveness of catalyst systems for polymerization of olefins based on use of aluminoxanes or modified aluminoxanes. In this connection, WO 2009/029857 shows dimethylaluminum cation formation from methylaluminoxane upon treatment of methylaluminoxane with a Lewis base. e.g., tetrahydrofuran, in a toluene solution. Lewis base stabilized dialkylaluminum cations can also be derived from non-aluminoxane sources and used as metallocene catalyst activators; see for example Klosin et al., WO 2000/011006, and Organometallics, 2000, 19, 4684-4686. When a methylaluminoxane is reacted with a metallocene, a metallocene-dialkylaluminum cation forms, for example, [Cp2Zr(μ-Me)2AlMe2]+ or [Cp2Ti(μ-Me)2AlMe2]+. See in this connection Babushkin and Brintzinger, J. Am. Chem. Soc., 2002, 124, 12869-12873, and Sarzotti et al., J. Polymer Sci. A, 2007, 45, 1677-1690, which describe activation of a zirconocene catalyst precursor by methylaluminoxane; also see Bryliakov, Talsi, and Bochmann, Organometallics, 2004, 23, 149-152, which describes activation of a titanocene catalyst precursor by methylaluminoxane.
Silica-supported methylaluminoxane is currently a preferred commercial metallocene catalyst activator. However, silica-supported methylaluminoxane can only activate a small amount of metallocene, resulting a relatively low efficiency for such systems. Low efficiency is believed to be caused by a need for a large excess of methylaluminoxane (e.g., an atomic ratio of Al:Zr greater than 400:1) to effectively activate the metallocene molecules, and/or by the small amount of methylaluminoxane that can be supported on standard grades of silica (e.g., <20% Al).
Improvements to aluminoxanes are continually sought in the art, particularly to increase their stability and/or activation efficiencies.